Microwave signals are transmitted from point-to-point by waveguides or antennas. The main difference between the two is that an antenna radiates the electromagnetic field into open space, while a waveguide confines the electromagnetic field to an area along the signal path. There are a number of conventional waveguide sub-categories.
A transmission line is one such sub-category. It uses some physical configuration of metal and/or dielectrics to direct a signal along the desired path. Typical transmission lines use two conductors—a signal and ground. There are also single conductor transmission lines, such as rectangular waveguides. The simplest type of transmission line configuration is coaxial line. Stripline is essentially a flattened version of the coaxial line configuration. Microstrip line simplifies the stripline configuration, by removing the upper ground planes.
Microstrip line is generally the most commonly used means for planar transmission line applications, because it is highly manufacturable and eases circuit connections and signal probing. Its disadvantage over stripline is that some of the energy transmitted may couple to adjacent traces or into space. Unlike microwave antennas, microwave waveguides are not intended to radiate energy in this way, which causes signal loss and interference. A coplanar waveguide with ground (CPWG) is essentially a low radiation version of microstrip.
In any case, as a microwave signal travels down the signal path of a waveguide, it is subjected to various characteristics associated with that path, such as path discontinuity. Such characteristics generally impact on the waveguides performance, and must be taken into account. For example, consider the case where a microwave signal must be routed from one side of a printed wiring board to the other side. This passing of the signal can be achieved with metallic pads on each side, and a hole that is plated with metal to connect these pads.
Such structures are commonly called vias. However, using a metallized hole and pad via structure to make a coaxial transition can create a non-uniform ground structure, which is associated with undesirable stray fields. In addition, the maximum frequency at which such via structures operate may be limited, depending on the particular application. A coax via structure solves such problems, where the coax is used to transfer signals through the board. However, the shield of the coax is traditionally connected to the ground plane of the board. This thickness of the shield is therefore part of the transition, and causes a discontinuity. Moreover, the thickness limits the minimum via hole diameter. Smaller diameter via holes may be desirable, but may adversely impact structural integrity of the via. In addition to the coax thickness problem, the pad to which the coax conductor is attached is much larger than the conductor, further contributing to transition discontinuity.
What is needed, therefore, is an improved coax via structure for transitioning high frequency signals on a printed wiring board.